How does MutationObserver let you react to DOM changes made by other code?

MutationObserver watches a target node (and optionally its subtree) for changes like added/removed children, attribute changes, or text changes, and asynchronously batches callbacks — useful for reacting to DOM mutations from third-party scripts you don't control.

const observer = new MutationObserver((mutations) => {
  mutations.forEach(m => console.log(m.type, m.target));
});
observer.observe(document.body, { childList: true, subtree: true });